In recent years, when an electrophotographic process is applied to form images, downsizing of toner particles is promoted in order to attain higher image quality, and a polymerized toner is manufactured to meet this demand. This polymerized toner is composed of toner particles prepared by coagulating constituent particles, for example: (i) resin particles prepared by conducting polymerization such as emulsion polymerization; (ii) colorant particles; and (iii) other particles if necessary.
So far, resin particles to be used in a polymerized toner have been prepared through emulsion polymerization in which oil droplets are formed, first, by dispersing a polymerizable monomer (raw material) in an aqueous medium containing an emulsifier, and then a polymerization initiator is added to conduct radical polymerization in each oil droplet. For example, a styrene-acryl copolymer has been prepared through this method (refer to Patent Document 1 and Patent Document 2, for example).
However, since polymerizable monomers used for radical polymerization are limited in such a toner manufacturing method, the resulting toner is also limited to those containing vinyl based resin particles or acryl based resin particles.
A polyester resin provides a toner exhibiting an excellent fixability in relation to a transfer material since the polyester resin exhibits an excellent viscoelasticity due to its high crystallinity and hardness. Alternatively, a styrene-acryl copolymer provides a toner exhibiting an excellent low-temperature fixability since it is easily softened at a lower temperature because of the low softening temperature due to its non-crystalline nature. Desired is a toner containing both the polyester resin and the styrene-acryl copolymer resin so that the toner simultaneously exhibits both the good properties of these resins. In order to obtain such a toner, proposed is a method to mix, melt and knead these resins (for example, refer to Patent Document 3).
However, in the conventional method in which the polyester resin and the styrene-acryl copolymer resin are simply melted and kneaded together, it has not been fully easy to homogeneously mix these resins each other due to a relatively large difference in the chemical structures of these resins. As a result, separation of these resins has occurred when pulverized, or the chemical compositions of the pulverized particles have become largely different and the distribution of the electrostatic charge of the toner has become broadened, resulting in causing defective images, for example, fogging or dispersal of toner, which are often observed when electrostatic charge of the toner is relatively small.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2000-214629
(Patent Document 2) JP-A No. 2001-125313
(Patent Document 3) JP-A No. 6-3856.